Field of the Invention
The invention relates to a method of setting the volumetric gas flow fed through a gas feed line to a burner nozzle of a gas-operated cooking or baking appliance and required for a desired burner heat output or heating power. The invention also relates to a corresponding cooking or baking appliance, in particular a gas cooker, a gas stove or gas oven, having a burner, the nozzle of which is connected to a gas feed line, and a gas valve arranged in the gas feed line.
Conventional gas-operated cooking or baking appliances, for example gas stoves, gas cookers, gas hobs, gas ovens, have one or more burners, in which gas is mixed with air and burned. City gas or natural gas, liquid gases (butane, propane) or gasified liquids (e.g. benzine, spirit) may serve as fuel. The gas types differ in their combustion behavior and their calorific value. The respectively used gas is extracted from a supply device, e.g. a gas supply network, a gas cylinder, a gas tank or reservoir, and fed to a burner via a gas feed line.
The feeding pressure of the gas relative to the atmosphere may vary depending on the gas system used (i.e. on the type of supply device and the type of gas used). But the gas pressure may also drop or increase by several mbar within a system.
The burner has a burner nozzle, which during operation of the burner constitutes the decisive flow resistance limiting the outflowing gas flow. The burner nozzle thus determines the maximum volumetric gas flow, i.e. the maximum heat output or heating power of the burner.
In the prior art, a proportional gas valve that is manually or electromotively actuated is used to set the desired burner heat output. The setpoint input to change the valve position is effected by means of a selector element, for example by turning an adjusting knob or by selection on a control panel. By partial opening or closing of the gas valve, the gas flow through the gas feed line (i.e. the volumetric gas flow through the line cross section per unit of time) is changed and thus the desired burner heat output is set.
A problem with such a gas valve is that it exhibits hysteresis in the setting behavior, i.e. the gas flow at the same valve position varies as a function of the length of the preceding adjusting travel and the direction in which the valve is actuated (i.e. opened or closed) for setting the desired gas flow. In addition, the control behavior of the gas valve may partly change to a considerable extent, depending on which gas type is used or on which burner type is connected to the gas feed line. The burner types differ, for example, in their maximum heat output, in their nozzle configuration, etc. For this reason, the burner nozzle and other parts must generally be exchanged if the cooking or baking appliance is to be operated with another gas type and/or another burner type.
On account of the hysteresis of the gas valve and gas-pressure fluctuations on the part of the gas-supply device, the reproducibility and setting accuracy of the volumetric gas flow is very slight if a user of the cooking or baking appliance is guided by a scale, assigned to the valve, of the selector element when setting the desired burner heat output. No accurately defined burner heat outputs can be selected by means of the scale. For this reason, the user generally pays only fleeting attention to the position of the selector element or to the scale. The user changes the position of the valve until the desired burner heat output is reached, in the course of which the user is in essence guided by the flame size and/or the cooking or baking behavior of the respective food.
German published patent application DE 196 27 539 A1 discloses a method and a configuration that sets the gas flow to a burner nozzle in steps. To this end, a plurality of choke elements are arranged in series or in parallel in the gas feed line. Binary switching elements are provided in order to switch the function of the respective choke elements on and off. The binary switching elements can switch the gas flow on and off by the choke element. A stepped reduction of the gas flow is carried out by the appropriate combination of certain switching elements in on or off position.
European published patent application EP 0 562 538 A2 describes a method and a corresponding appliance for setting and keeping constant a volumetric gas flow corresponding to a desired burner heat output. There, the pressure of the gas fed to a burner nozzle is measured by means of a pressure sensor and is brought to a predetermined value or is kept at the predetermined value by means of a control loop.